This invention relates to an ABS braking system for motor vehicles.
Typically, production vehicles, such as automobiles having a Automatic Braking System (ABS) employ a single brake disc fixed to the wheel hub and a sliding brake caliper mounted on a suspension member of the vehicle. Operation of a brake foot pedal by the operator with excessive force is sensed by a sensor, and a deceleration of the wheel is sensed by a wheel position or speed sensor mounted adjacent the wheel. The wheel speed sensor particularly monitors for front wheel locking up while in the braking condition with its loss of steerability and its longer stopping distance. The stopping distance of the vehicle may be made shorter if the wheels are operated iteratively at a low slip rather than a longer, fully locked or skid condition. The brake caliper is preferably operated at a high brake torque xe2x80x9capplyxe2x80x9d rate to increase brake torque for quick response. Additionally, caliper preferably is operated at a xe2x80x9chigh releasexe2x80x9d rate to decrease brake torque for quick response when the condition of lock-up is sensed as about to begin to allow the wheel to accelerate to a velocity approaching the vehicle velocity.
In conventional production vehicles having this fixed brake disc and slidable brake caliper, a hydraulic piston in a hydraulic cylinder on the caliper is operated to shift an inner movable brake pad into braking engagement with one side of the brake disc fixed to a rotating wheel hub mounted on the vehicle suspension. A reaction force from the hydraulic fluid moving the piston shifts the slidable caliper to slide the caliper and a second brake pad on the distal end of a caliper into engagement with the other side of the fixed brake disc. Typically, the braking system operates with hydraulic fluid at a pressure of about 70 BAR or more to provide the clamping pressure to opposite sides of the brake disc.
From the foregoing, it will be understood that when high excessive force is applied to a brake pedal by a vehicle operator causing rapidly deceleration of the wheel towards the lock-up condition, the ABS sensors and control system senses the rotational position of the wheel relative to the vehicle""s speed; and if these conditions are within preset stored parameters, the ABS hydraulic system is activated to operate the brakes. The ABS hydraulic system isolates the pedal-operated hydraulics, and the braking operation is taken over by the ABS system, which causes the braking effort to drop and allows the wheel rotation to accelerate or spin up. When the wheel spin-up approaches the vehicle speed, as sensed by the wheel sensor, but does not equal the vehicle speed, the ABS hydraulics apply increased hydraulic pressure to the sliding caliper to decrease or spin down the braked wheel""s rotational velocity. When the vehicle wheel spins down to approach vehicle lock as sensed by the wheel sensor and within the preset parameters, the ABS hydraulic pressure at the slidable caliper is increased to allow wheel spin up. This process is iterated to provide modulation of the ABS hydraulic pressure and a deceleration of the vehicle""s velocity to provide a stopping of the vehicle within a predetermined distance depending on the kind of surface on which the wheels are engaging. Manifestly, the stopping distance on ice or other low coefficient of friction surfaces is greater than the sloping distance for higher coefficient of friction surfaces. Governmental regulations in many countries require the ABS braking system to stop the vehicle within a set stopping distance for a given coefficient of friction surface.
The ABS system senses the initial apply rate and release rates and scales or calibrates the resolution of subsequent xe2x80x9capplyxe2x80x9d and xe2x80x9creleasexe2x80x9d rates to stop the vehicle. For example, the amplitude of the initial apply and release rates is quite high when the wheel is on a dry concrete surface and the frequency of the brake applications and releases is quite large in amplitude and at a low frequency. The ABS system then compares the frequency and amplitude of brake apply and brake release rates relative to the preset stored parameters in a controller and then operates the braking system according to this algorithm that is used to decelerate the vehicle""s speed to stop the vehicle with the governmental stopping distance. This deceleration is usually a constant deceleration and is linear with respect to time and vehicle velocity or distance traveled. Hereinafter, this will be called a vehicle deceleration curve, which need not be linear, but which is usually a linear curve. A graphical representation of vehicle wheel speed and of hydraulic pressure shows that they are modulated along this theoretical vehicle deceleration curve until the vehicle is slowed down to a very low speed, e.g., under 10 mph when the brakes are allowed to lock up to complete the stopping of the vehicle. If the same vehicle traveling at the same speed and braked with the same brake pedal pressure was traveling on polished ice, having a very low coefficient of friction, relative to the coefficient of friction for the concrete surface, the initial braking by the ABS system recognizes this and sets the scale or calibration to generate a finer resolution with a more frequent application and release of the brakes and with a smaller amplitude of wheel acceleration and deceleration. Thus, with a smaller coefficient of friction surface, the average pressure variation and wheel acceleration is less over the stopping distance.
Current ABS braking systems suffer from being relatively heavy in weight, in being relatively costly, and from operational deficiencies such as operating at high pressures, high residual torque drag, and large hystersis losses. The present invention is directed to providing a significant weight reduction, for example, with respect to one commercial automobile, to reduce the weight of the braking system from about 18 kilograms to 15 kilograms of unsprung weight at the wheel. A cost saving of thirty (30%) percent or more can be achieved relative to the braking system currently used on a commercial vehicle. As will be explained in detail hereinafter, better operation is achieved with a reduction in residual torque drag, which occurs when the brake pads rub against the brake disc when the brake pedal has not been operated. A reduction in residual torque drag results in a significant increase in brake pad life, lower operating temperatures, and faster wheel acceleration as will be described in more detail hereinafter.
To facilitate acceptance and adoption of the braking system of this invention by original equipment manufacturers, the preferred braking system of this invention may be used with the same installed ABS controller and operating system on vehicles now in use. The preferred braking system without the ABS control system is more fully described in the aforesaid co-pending United States patent application and to a lesser extent hereinafter in this application. This braking system includes twin slidable brake discs and four brake pads for rubbing or clamping engagement with the four sides on the twin brake discs and a fixed caliper mounted on a suspension stub axle or knuckle. Preferably, the hydraulic cylinder for operating the brake discs is formed integrally with the suspension knuckle; and an outer, distal brake pad is fixedly mounted on a stationary bridge of the caliper. This is unlike the current slidable caliper on the typical conventional disc brake that has only two rubbing surfaces engaging opposite sides of a cast iron or cast aluminum, heavy brake rotor.
The present invention is, as stated above, directed to providing a better ABS system from an operational standpoint. Current ABS systems suffer from a number of shortcomings. One of these shortcomings is that they operate at relatively high hydraulic pressures, for example, 70 BAR on high friction surfaces. With only two braking surfaces for a wheel, a large amount of energy must be dissipated at each rubbing surface to decelerate the vehicle quickly. These high brake pressures result in large amplitude variations in hydraulic pressure when the brakes are being applied and released, During the long period of time that the brake pressure is released to allow wheel acceleration toward the vehicle""s velocity, the vehicle is moving forwardly with no braking effort being applied to decelerate the vehicle.
With a finer resolution of the frequency and amplitude about a theoretical, vehicle braking curve, the time for an individual cycle of brake application and brake release is much quicker so that many more of these cycles are performed in the same period of time than for higher resolution braking system. Hence, it would be desirable to have an ABS system with increased resolution ore closely approximating the vehicle""s deceleration curve. Another factor involved in the obtaining of better braking and, also in obtaining finer resolution of braking and releasing cycles is that of the hystersis of the system, which involves wasted energy and time put into the system. For example, in the current hydraulic systems there are expandable, flexible, hydraulic hoses or lines that expand during the high pressure braking and contract during the lower pressure release of the brakes. Also, several seals are expanded during high pressure braking and then contract during the pressure release. One such seal in a hydraulic braking system is the annular seal about the piston of the brake caliper""s cylinder. This seal is expanded during braking and contracts during brake release and exerts a return force to return the piston.
Another operational shortcoming of the commonly used slidable caliper, single disc braking system is the amount of deflection of the distal brake pad""s support at an outer end of the caliper. That is, in the current disc braking system, the large, heavy sliding caliper has a fixed pad on the caliper which is that outer end of a caliper bridge. This caliper and its bridge are large and heavy because they carry the piston and the outer distal pad and provide the stiffness needed to resist bending and deflection from the large clamping forces being applied. Despite its being relatively heavy and large, the caliper""s distal end often deflects about 0.0006 inch or greater. The operation in the system is affected by this deflection of the caliper""s distal end and the time needed to slide the heavy caliper back to its brake release position.
As stated above, the operational performance of an ABS disc brake system is adversely affected by residual drag of the brakes when the braking pressure has been released. Residual braking torque retards wheel spin up to the desired speed, and thus, slows the time of response. It has been found that the conventional slidable caliper brake disc herein described has significant residual torque or brake drag. One cause may be that the brake disc is fixed to a stub and whatever tolerance it has from a true perpendicular relationship to the rotational axes of the wheel results in rubbing of the disc at high spots, which is called xe2x80x9crun outxe2x80x9d. That is, the fixed disc or rotor will not run absolutely true because you have manufacturing tolerances in its support including bearings, hubs or axles, and in the disc itself, which is cast with angular portion therein. This rotating fixed rotor has a geometry envelope within which its annular braking surface travels during a wheel revolution. When a high spot on the rotor hits or rubs against a brake pad, it pushes against the high mass, caliper, and residual torque drag is the result. This reduces the life of brake pads and wastes fuel and energy. Also, as the wheel is released by the ABS system to accelerate toward the vehicle velocity, the wheel must overcome this residual torque drag as it accelerates. This residual torque drag prolongs the time needed to reach the desired wheel velocity and thus, increases the stopping distance for the ABS braking system. That is, the higher hydraulic operating pressures and mass of the sliding caliper and associated friction losses of this sliding caliper system result in more time betweens spin down and spin up. Hence, it would be desirable to provide a more effective ABS system wherein the frequency of spin up and spin down is faster.
To provide an effective ABS braking system, the brake system itself must pass rigorous specifications for wear, vibration, residual torque as well as various road tests for brake fade, for temperature of operation on mountainous descents or curvy roads over long period of time, etc. Some of the current brake systems using two brake pads and a single fixed brake disc operate at such high temperatures they either fail or are having difficulty in passing the Auto Motive Standard (AMS) road test.
Additionally, for a brake system to be installed on production vehicles, it must operate successfully and be free of vibrations, noise or other adverse feel conditions that are deemed undesirable by the vehicle operator. Of course, longevity of the brake pads and discs with a minimum of wear at localized areas that results in disc thickness variation (DTV) is most desirable to avoid vibrations and replacement of brake discs and/or brake pads. In an off-brake condition, the brake pads and brake disc can touch, particularly when cornering or traveling over bumpy surfaces and cause residual torque drag. This residual torque drag is additional to that above-described due to manufacturing tolerances.
In accordance with the present invention, there is provided a new and improved ABS braking system for vehicles that is lighter in weight, lower in cost and has improved operating characteristics. To achieve these ends, the preferred ABS system is formed with a pair of slidable, thin brake discs, which are mounted in a floating manner on a wheel hub and this floating allows the brake discs to position themselves quickly and with reduced, residual brake torque. From a weight standpoint, these brake discs are thin, flat plates made of steel, aluminum or a composite material as contrasted to the heavy cast, angular rotors currently used. The hydraulic brake cylinder is integrally formed inside the stub axle or knuckle to provide a significant weight savings. From an operational standpoint, the ABS system works at substantially reduced hydraulic pressures and has a higher frequency of wheel deceleration and accelerations, and lower amplitude of pressure and wheel velocities than that of the conventional ABS system. Thus, there is provided an ABS braking system operating at a lower pressure, with reduced hystersis, and improved modulation of braking relative to a conventional, production ABS system which was a slidable caliper and a pair of brake pads cooperable with a single, fixed brake disc.
This is achieved by employing with the ABS electronic controller systems a braking system using four rubbing or braking surfaces and two slidable discs, a lighter and smaller caliper, greater caliper stiffness, reduced hystersis and lower operating forces.
In accordance with the invention, the preferred ABS braking system, when used on a conventional vehicle having a standard ABS controller, develops from the initial application of the braking application a resolution of modulating scale for cycling that is associated with a much lower coefficient of friction surfaces than is actually present. That is, the same ABS controller on a high coefficient of friction road surfaces operates the conventional, fixed, single brake disc system in the manner associated with this high coefficient of friction surface but operates the twin, slidable brake disc system in the manner associated with a much lower coefficient of friction, road surface. This is a result of using the slidable pair of brake discs and four brake pads operating at about one half of the usual hydraulic pressure and having reduced residual torque drag and hystersis such that the initial cycles used to scale or calibrate the modulators is one half or less than that used for the conventional sliding caliper and fixed disc kind of system. Stated differently, when using a conventional ABS system controller and hydraulic master cylinder system connected to the preferred turn, slidable base discs and four brake pads, the system will provide more than double the number of braking and release cycles per unit time. The braking and release cycles will have a much higher frequency and a substantial less amplitude meaning that the vehicle will be decelerating more closely to the desired vehicle deceleration curve set by the ABS controller.
In accordance with an important aspect of the invention, the slidable brake pads are mounted upwardly at about a 12:00 o""clock position with a wheel speed, pulse generator located adjacent thereto so as not to interfere with the locking angle and turning circle of the vehicle suspension. Preferably, the hydraulic cylinder is integrally formed at the top of the suspension member. Also, it is preferred to provide the seal ring between the piston and the cylindrical wall of the cylinder with a low friction surface, such as a Teflon surface, to reduce hystersis.
In the preferred embodiment, the wheel speed pulse generator includes an annular air cooling ring attached to the rotatable hub with equally spaced tabs arranged in a circle concentric with the wheel axle. A magnetic sensor is mounted on the suspension member adjacent the fixed caliper to sense the spaced tabs and thereby, the rotational speed of the wheel relative to lock-up and/or vehicle speed.